Staling of baked foodstuffs (such as bread) is a well-known problem. Staling, or “going stale”, is a chemical and physical process in baked foods that reduces their palatability. Staling becomes evident as an increase of the firmness of the crumb, a decrease of the elasticity or resilience of the crumb, and changes in the crust, which becomes tough and leathery. The increase in crumb firmness, which is considered as the most important aspect of staling, is recognized by the consumer a long time before the bread product has otherwise become unsuitable for consumption.
Staling is not, as is commonly believed, simply a drying-out process due to evaporation of water. Bread will stale even in a moist environment and stales most rapidly at temperatures just above freezing. Although the precise mechanism of staling is still unknown, one important mechanism appears to be migration of moisture from the starch granules into the interstitial spaces, and realigning of amylose and amylopectin molecules of starch. The process of the realignment of the starch molecules is called retrogradation. On retrogradation, crystal-like structures may form that are similar to those originally present in the starch granules, and the process is referred to as recrystalisation. Retrogradation normally results in stale bread with a harder, less resilient crumb texture.
Starch is an essential constituent of baked foodstuffs. During the baking process, the starch becomes gelatinized and absorbs large amounts of water; meanwhile the protein denatures. Immediately after baking, the starch begins to retrograde. The firmness of the crumb increases, although this is still regarded as an advantage in the first hours. In particular the sliceability and chewing characteristics of the crumb improve during this period.
It is assumed that the unbranched starch fraction, amylose, retrogrades first, followed by the branched fraction of the starch, amylopectin, during further storage. At the same time the crumb becomes stiffer, and in the course of time increasingly less elastic and eventually dry and hard: the bread has become stale. In addition, the crust looses crispness and becomes leathery during storage. It is assumed that this is a result of water being released by retrogradation and diffusing outward from the crumb to the crust.
It is undisputed that the causal key reaction for all of these staling phenomena is starch retrogradation. Suppressing or circumventing this phenomenon is the subject matter of numerous protective rights and publications.
One strategy for hindering, at least partly, the considerable firming of the crumb during storage has already been long known: the crumb is made softer from the beginning. The means of choice to do this are emulsifiers such as mono/diglycerides, which are added to the dough and produce a crumb structure that is particularly soft from the beginning. The use of α-amylase derived from fungi such as Aspergillus oryzae has a similar effect. It acts upon damaged starch particles, thereby lowering the viscosity of the dough and producing feimentable sugars. As a consequence, the finished baked article has larger volume, which is consistent with softer crumb. Aside from the fact that the fresh bread is soft, this strategy does not prevent or inadequately prevents the development of a harder, less elastic consistency of the crumb when it becomes stale.
A further strategy is to reduce retrogradation by enzyme-mediated partial hydrolysis of the two starch fractions during baking. Enzyme-mediated hydrolysis of the crumb should preferably take place after the starch has been gelatinized, i.e., above about 65° C. As a consequence the structure of the starch in the baked product is radically altered, limiting its ability to retrograde. The fragments produced by partial hydrolysis of the starch are too short to be able to recrystallize and associate with the remaining high molecular weight starch, thereby reducing speed of recrystallization. A heat-stable maltogenic amylase from Bacillus is commercially available under the trade name Novamyl 10,000® (product of Novozymes A/S, Denmark) and is widely used in the baking industry as an anti-staling agent due to its ability to reduce retrogradation by hydrolyzing starch at starch gelatinization temperature (WO 91/04669). Novamyl 10,000® is most active at 60-70° C. (Christophersen, C., et al., 1997, Starch, vol. 50, No. 1, 3945).
Since significant costs are associated with the use of maltogenic amylase as an anti-staling agent in baked bread, there is a need for a more cost-effective anti-staling agent.